Conventionally, a medical tube used for a catheter, an infusion bag, a blood bag, or the like has been often manufactured by using soft vinyl chloride resin excellent in tube characteristics such as flexibility, transparency, kink resistance, and clamp resistance. However, it has been pointed out that soft vinyl chloride resin has a safety problem in that a plasticizer having a lower molecular weight is eluted, and has an environmental problem in that a toxic gas is produced at the time of its disposal and incineration.
Thus, in order to avoid the above-mentioned problems, the applicant proposed, as a resin composition having flexibility and being suitable for medical application, a resin composition produced by blending a polypropylene-based resin with a hydrogenated elastomer of a block copolymer containing a polymer block formed of an aromatic vinyl compound and a conjugated diene-based polymer block (see Patent Literature 1).
A tube made from this thermoplastic elastomer has had transparency and flexibility, but has had points to be improved in kink resistance, clamp resistance, and anti-conglutination property.
In order to improve kink resistance, it is known to produce a tube by blending a polyolefin-based resin with a specific hydrogenated styrene-based thermoplastic elastomer, a softening agent, and the like (see Patent Literatures 2 and 3). As far as anti-conglutination property concerned, there is known a soft polymer composition including a block copolymer formed of polystyrene and an ethylene-propylene copolymer, a block copolymer formed of polystyrene and an ethylene-butylene copolymer, and a polyolefin (see Patent Literature 4).
Further, in order for a tube to have a plurality of characteristics, which are required to serve as a good tube, it is known to adopt a multilayered tube having a surface resin layer and an inner resin layer formed of different compositions (see Patent Literatures 5 and 6).
In addition, there is known a tube for medical purpose including a resin composition produced by blending (a) a hydrogenated block copolymer prepared by hydrogenating a polymer block formed of a vinyl aromatic compound and a polymer block formed of a conjugated diene, (b) an olefin-based copolymer, and (c) a hydrocarbon-based softening agent for a rubber (see Patent Literature 7).    [Patent Literature 1] JP 10-67894 A    [Patent Literature 2] JP 2002-248671 A    [Patent Literature 3] JP 2003-287163 A    [Patent Literature 4] JP 04-159344 A    [Patent Literature 5] JP 2001-1432 A    [Patent Literature 6] JP 2004-194803 A    [Patent Literature 7] WO 2006/134974 A1